Print this pageDiagenetic Facies in Lacustrine Carbonates: Implications for Brazilian Pre-Salt Reservoirs
Recent hydrocarbon discoveries in offshore Brazil have generated great interest in lacustrine carbonate reservoirs. Currently, there are significant gaps in industry knowledge regarding lacustrine carbonate environments and, especially, their subsequent diagenetic alteration processes. Improved reservoir quality prediction in these lacustrine carbonates requires a greater appreciation and resolution of the diagenetic alteration spatial distribution.
Initial reservoir quality in lacustrine deposits is highly variable. For example, the dominance of ubiquitous microbialite textures (e.g., microbial “shrubby” precipitates) in these settings often results in poor depositional porosity. Lakes are also prone to frequent changes in water level, imparting high-frequency variability on the resulting accumulation. As a result, lacustrine microbialites are typically thinly bedded, with vertical heterogeneities over centimeter- to millimeter-scales. Worldwide, microbialites often require diagenetic enhancement to form an adequate petroleum reservoir.
In general, lacustrine microbialites exhibit a complex paragenesis. Several burial diagenetic processes have been recognized and appear to provide the greatest modifications to lacustrine microbialite pore networks. These processes are: 1) fracturing, 2) metallic sulfide mineralization, 3) dolomitization, and 4) dissolution. The importance of burial diagenetic processes can further be substantiated by seismic images/attributes which may provide valuable insights into fault damage zones, disrupted strata, and cavernous porosity distributions. In these buried lacustrine reservoirs, fault/fracture networks are believed to be the critical pathways for delivering fluids, thereby acting as diagenetic facilitators. Fractures provide avenues of communication between exotic fluids derived from aquifers in volcaniclastic/igneous rocks and the overlying lacustrine carbonates. Significantly, dissolution likely provides the greatest uplift in reservoir quality through connected vuggy porosity networks. Preliminary observations of these vuggy porosity networks indicate formation after stylolitization, further reinforcing a burial origin. Based on this robust diagenetic alteration, conceptual lacustrine diagenetic models that unify all the elements of the burial processes are likely to be the best predictors of zones of favorable reservoir quality.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil